This invention relates to a method for making an optical waveguide fiber with a fatigue resistant TiO.sub.2 --SiO.sub.2 outer cladding.
Although glass is a brittle material, the intrinsic strength of pristine glass optical fibers is very high, on the order of 1,000,000 psi for SiO.sub.2 based fibers. Typically, glass optical fibers fail from surface imperfections when placed under sufficient tensile stress. Accordingly, much effort has been devoted to the elimination of surface flaws by careful handling during and after glass forming, by a protective plastic coating, and by various treatments to the glass surface. In the latter case, one method of reducing failure by surface flaws is to provide a compressive stress on the glass surface that counteracts applied tensile stresses.
It is well known that flaws in glass grow subcritically prior to failure when subjected to tensile stress in the presence of water, ammonia, or other corrosive agents. This phenomenon of subcritical crack growth in glass is known as fatigue and greatly impacts the long-term reliability of glass based materials such as glass optical fibers. Therefore, the fatigue performance of optical fiber is especially important to the design of low cost fiber cables which have fewer strength members and less environmental protection than standard optical telecommunications cables.
It has been known for some time that the strength of a glass body may be increased by forming its surface region from a glass with a thermal coefficient of expansion that is lower than the thermal coefficient of expansion of the interior glass. As the combination is cooled from high temperatures, this configuration places the glass surface in compression, thereby inhibiting the formation and growth of cracks. See, e.g.: Giffen et al. U.S. Pat. No. 3,673,049; and Krohn and Cooper, "Strengthening of Glass Fibers: I, Cladding", Journal of the American Ceramic Society, Vol. 52, No. 12, pp. 661-4, December 1969.
Numerous attempts have been made to create a strengthened optical fiber with such a compressive surface layer. See, Maurer et al. U.S. Pat. No. 3,884,550; MacChesney et al., "Low Loss Silica Core-Borosilicate Clad Fiber Optical Waveguide", American Ceramic Society Bulletin, Vol. 52, p. 713, 1973. Macedo U.S. Pat. No. 4,181,403 refers to compression in a thin surface layer formed by "molecular stuffing" in fiber with a large optical core and very thin optical cladding. Some of these attempts involved the use of a TiO.sub.2 --SiO.sub.2 outer layer on the fiber, as its thermal coefficient of expansion is known to be less than that of SiO.sub.2. See, e.g.: Schneider et al. U.S. Pat. No. 4,184,860; Kao et al. U.S. Pat. No. 4,243,298; and, Taka et al. Japanese Patent No. 1,255,795.
TiO.sub.2 --SiO.sub.2 layers have also been shown to result in higher breaking stresses and increased fatigue resistance, a measure of a material's susceptibility to subcritical crack growth under stress (fatigue resistance is further defined below). See, e.g.: Oh, Predieux and Glavas, "Increased Durability of Optical Fiber Through the Use of Compressive Cladding", Optics Letters, Vol 7, No. 5, pp. 241-243, May 1982. In addition, Corning Incorporated, the inventors' employer, has sold prior art fibers which included a cylindrically uniform 3 .mu.m (.+-.0.5 .mu.m)] outer cladding layer with a substantially homogeneous glass composition of 8 wt. % TiO.sub.2 (.+-.2 wt. %) in an SiO.sub.2 matrix. The performance of these fibers is described in Glaesemann and Gulati, "Dynamic Fatigue Data for Fatigue Resistant Fiber in Tension vs. Bending", OFC Conference, 1989 Technical Digest Series, Vol. 5, WA3, February 1989, and Gulati et al., "Improvements in Optical Fiber Reliability via High Fatigue Resistant Composition", SPIE, Vol. 842, Fiber Optics Reliability: Benign and Adverse Environments, pp. 22-31, 1987. These fibers had fatigue resistance "n" values in the range of 26-32 (n is defined below).
These prior art references typically provide a bulk examination of the compressive layer, i.e., by reference to its thermal coefficient of expansion mismatch with the interior of the fiber. Japanese patent No. 1,255,795, for example, postulates that SiO.sub.2 --TiO.sub.2 glasses with up to 25 mol % TiO.sub.2 (30.8 wt. %) may be used in the outer cladding, stating that the thermal coefficient of expansion of TiO.sub.2 --SiO.sub.2 glass is increasingly negative until this percentage is reached.
Schneider et al. U.S. Pat. No. 4,184,860 describes an outer TiO.sub.2 --SiO.sub.2 layer with 8 wt. % TiO.sub.2 surrounding a 15 wt. % TiO.sub.2 layer which is heat treated (by "tempering") to devitrify and partially separate and/or crystallize. This heat treatment of the 15 wt. % TiO.sub.2 intermediate layer is intended to raise the thermal coefficient of expansion so that it is substantially greater than the coefficient of the outer TiO.sub.2 --SiO.sub.2 layer, thereby putting the outer layer in compression. Thus, the Schneider et al. fiber design relies on the 8 wt. % TiO.sub.2 outer layer to provide enhanced strength through compression.
Schultz studied SiO.sub.2 --TiO.sub.2 glasses containing 10-20 wt. % TiO.sub.2 which were clear when formed, but which exhibited increased opacity from phase separation and anatase formation, along with large changes in thermal expansion, upon heat treatment at temperatures below the annealing point. "Binary Titania-Silica Glasses Containing 10 to 20 Wt. % TiO.sub.2 ", Journal of the American Ceramic Society, Vol. 58, No. 5-6, May-June 1976 (Schultz U.S. Pat. No. 3,690,855). By studying the physical properties of these TiO.sub.2 --SiO.sub.2 compositions, Schultz described three glass forming regions as stable (0-10 wt. % , metastable (10-18 wt. %) and unstable (&gt;18 wt. %).
Some recent research has been directed toward understanding the mechanism of crack growth in SiO.sub.2 glass on the molecular level. See, Michalske and Bunker, "The Fracturing of Glass", Scientific American, December 1987, pp. 122-129. The Michalske and Bunker paper presents an atomistic study of glass fracture in the presence of water, but is limited to homogeneous SiO.sub.2 glass. Additional research has been directed toward crack growth in continuous fiber filled composites. See, Michalske and Hellmann, "Strength and Toughness of Continuous-Alumina Fiber-Reinforced Glass-Matrix Composites," Journal of the American Ceramic Society, Vol. 71, No. 9, pp. 725-31, September 1988.